Life on Earth obtains energy in many types of formats. The two main process are photosynthesis and respiration. Photosynthesis is the name of the process by which autotrophs (self-feeders) convert
water, carbon dioxide, and solar energy into sugars and oxygen. Oxygen is a waste product of this activity. The photosynthetic activity of early bacteria helped build the oxygen content of Earth's atmosphere so that oxygen-dependent animals, including humans,
could live.

The reverse of this process, used by heterotrophs (other-feeders) coverts
sugars and oxygen into biological energy. Carbon dioxide and water are produced as waste products.

Respiration, unlike photosynthesis, takes advantage of a natural tendency of oxygen to combine with other molecules. When combining with other substances, oxygen tends to release large amounts of energy. Thus life forms which employ the process of respiration are taking advantage of the most efficient and energetic pathway nature provides to produce bioenergy on Earth.

Certain forms of bacteria can covert energy by breaking down exotic molecules without the benefit of photosnythesis or respiration, however. An example is the coversion of acetate to methane by Archaea. Likewise, life elsewhere in the solar system may rely on an energy conversion process which has yet to be discovered by human science. One way to search for life is to try and detect the waste products from life's activity. Read more about NASA's current attempts to search for life in the Exploratour on NASA's Exploration for Life at the bottom of this page.

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